The cardiovascular adjustments that occur during pregnancy include chronic increases in cardiac output, falls in total peripheral vascular resistance and tachycardia. There is an upregulation of endothelial nitric oxide synthase in almost all vascular beds studied in the gravid female including skeletal muscle, kidney and utems or placenta. The upregulation of eNOS directly contributes to the fall in TPR which is not confined to the placenta. Many studies have investigated the role of NO in the control of vascular resistance or how NO may buffer vasoconstriction and that a defect in NO production may be involved in pre-eclampsia. Despite increasing evidence that NO also modulates mitochondrial metabolism and substrate uptake by the heart, i.e. prevents glucose uptake and facilitates fatty acid uptake, there are literally no studies that have investigated the role of increased eNOS in the control of substrate uptake and organ oxygen consumption at all. We have previously shown that NO by interacting with cytochrome oxidase in heart, kidney and skeletal muscle serves to maximize the ratio of oxygen consumed to external work performed ie. increases efficiency. We have also shown that when eNOS produces NO in the heart and elsewhere, glucose uptake is prevented. It is important to re-emphasize that pregnancy is characterized by increased eNOS gene expression and increased NO production in every vascular bed of the mother. Furthermore, glucose uptake by the mother is low even insulin insensitive and this is thought to increase the amount of glucose available for uptake through the placenta to support fetal metabolism, since the placenta does not take up fatty acids. In addition a small but significant number of mothers go on to have a post-partum cardiomyopathy often leading to heart transplantation, perhaps when adjustments that occur during pregnancy do not regress after parturition. Thus the focus of this competitive renewal application will be the role of NO in the control of oxygen and substrate use during pregnancy with particular reference to the heart and coronary circulation. In the first specific aim, we will examine the role of NO in the control or metabolism in aged eNOS KO mice. The second aim will focus on the role of NO in the pregnant eNOS KO mouse whereas the third specific aim will focus on the role of NO in cardiac glucose and oxygen uptake in the rat heart during pregnancy. Finally aim 4 will use chronically instrumented conscious pregnant dogs to address the role of NO in the control of cardiac function, substrate use and oxygen consumption during pregnancy and after parturition. For the first time we wilt perform a systematic mechanistic investigation into the role of NO in the control of cardiac oxygen and substrate use during pregnancy. These studies have direct application to the physiology of pregnancy and to the potential mechanisms resulting in post partum cardiac dysfunction.